The cars or cages of electric elevators are moved by means of bearer or hoisting ropes. Where a drum drive is used, the car is moved by winding and unwinding the bearer rope around a drum provided with spiral grooves. Such drum drives are for the most part only permitted for elevators without counterweights and only for operating speeds of up to 0.5 m/s. In the case of pulley drives the bearer rope is carried along by the drive pulley by friction contact. The elevator car is fixed to one end of the bearer rope and the counterweight to the other end. The car and the counterweight run in guides. The counterweight normally amounts to the weight of the car plus half the permitted load.
Now if the car is empty or only lightly loaded it may happen, for example as a result of breakage of the ring gear transmitting the driving and braking forces to the driving pulley of a pulley drive, that the driving pulley and its shaft are free to rotate. The excess weight on the counterweight side of the bearer rope then pulls the car up at ever-increasing speed, and the acceleration when the weights are not in balance is further progressively increased by the lower part of the rope as the weight of the bearer rope is transferred from the car strand to the counterweight strand of the bearer rope.
The resulting uncontrolled upward movement of the elevator car, which is also known as an "upward crash", can cause accidents if the arresting device prescribed by the safety regulations is only effective in the case of upward travel.
An elevator brake aimed at preventing an upward crash, having brake jaws that act on at least one bearer rope, is known from The Netherlands patent specification 80 017. If the speed of the bearer rope, either upwards or downwards, exceeds a limiting value the brake is applied in a speed-dependent manner.